Excitation Polarization Research Articles

Interplay of Quenching and Enhancement Effects in Apertureless Near-Field Fluorescence Imaging of Single Nanoparticles

We systematically explore the interaction of an AFM tip with single CdSe/CdS quantum dots and seeded CdSe/CdS nanorods. Using distance-dependent intensity and lifetime near-field microscopy in 3D, we analyze the interplay between quenching and enhancement in proximity to the tip. Under tightly focused radially polarized excitation, a nanoscale, central enhancement spot is observed for both types of particles, revealing an identical physical mechanism underlying the near-field interaction in both cases. Furthermore, lifetime and intensity near-field images of both types of nanoparticles exhibit characteristics similar to those of a single molecule with a well-defined molecular dipole. We also investigate the origin of the observed enhancement effect. By exploring the dependence on excitation polarization and tip material, we conclude that the main contribution to the fluorescence enhancement is from excitation field enhancement at the apex of the tip, serving as a lightening rod. However, we also show clea...

Re-radiation Enhancement in Polarized Surface-Enhanced Resonant Raman Scattering of Randomly Oriented Molecules on Self-Organized Gold Nanowires

We explore the effect of re-radiation in surface-enhanced Raman scattering (SERS) through polarization-sensitive experiments on self-organized gold nanowires on which randomly oriented Methylene Blue molecules are adsorbed. We provide the exact laws ruling the polarized, unpolarized, and parallel- and cross-polarized SERS intensity as a function of the field polarizations. We show that SERS is polarized along the wire-to-wire nanocavity axis, independently from the excitation polarization. This proves the selective enhancement of the Raman dipole component parallel to the nanocavity at the single molecule level. Introducing a field enhancement tensor to account for the anisotropic polarization response of the nanowires, we work out a model that correctly predicts the experimental results for any excitation/detection polarization and goes beyond the E(4) approximation. We also show how polarization-sensitive SERS experiments permit one to evaluate independently the excitation and the re-radiation enhancement factors accessing the orientation-averaged non-diagonal components of the molecular Raman polarizability tensor.

Dynamic nuclear self-polarization with circularly polarized light

Lattice nuclei of III-V type semiconductors can be self-polarized through the contact hyperfine interaction with artificially excited conduction electrons at extremely low temperatures. In the self-polarization process, a positive feedback between the nuclear magnetization and the expansion of the energy splitting of the conduction electron states due to this nuclear magnetization takes an essential role. Extending the original theoretical work on the self-polarization with linearly polarized (LP) light by Dyakonov and Perel,1 we examined the effect of the circularly polarized (CP) light on the phenomenon of the nuclear self-polarization. While no polarization is obtained with LP light above the critical temperature, the nuclear polarization is enhanced also above the critical temperatures with CP light. More important finding in the application of CP light is that the direction of the nuclear polarization can be controlled by the orientation of the excited conduction-electron polarization, which is easily manipulated by changing the degree of polarization of the excitation light. Related to this polarization change, it is also remarked that the nuclear polarization shows a hysteresis phenomenon below the critical temperature when the degree of the electron polarization is swept up and down.

Photoluminescence of Liquid Crystal with Organic Thin Film of (2-(Naphtha[3,4]imidazol-2-yl) Quinoline)

A complex sample structure including nematic liquid crystals (NLC) with organic thin film (2-(naphtha[3,4]imidazol-2-yl) quinoline) (NIQ) is applicative to the photoluminescence (PL) study in this report. The pumping beams of 325 nm and 390 nm are used to excite the complex sample structure and the NIQ thin film. The excitation polarization about directorial orientation can cause the changing of absorption intensities in PL spectrum. Peak intensities can thus be modulated when the polarization of pumping beam adjusted. Consequently, the peak shifting in the PL spectrum of complex sample structure is revealed the energy transformation between NLC and NIQ.

Graphene Nanoribbons from Unzipped Carbon Nanotubes: Atomic Structures, Raman Spectroscopy, and Electrical Properties

We investigated the atomic structures, Raman spectroscopic and electrical transport properties of individual graphene nanoribbons (GNRs, widths ~10-30 nm) derived from sonochemical unzipping of multiwalled carbon nanotubes (MWNTs). Aberration-corrected transmission electron microscopy (TEM) revealed a high percentage of two-layer (2 L) GNRs and some single-layer ribbons. The layer-layer stacking angles ranged from 0° to 30° including average chiral angles near 30° (armchair orientation) or 0° (zigzag orientation). A large fraction of GNRs with bent and smooth edges was observed, while the rest showed flat and less smooth edges (roughness ≤1 nm). Polarized Raman spectroscopy probed individual GNRs to reveal D/G ratios and ratios of D band intensities at parallel and perpendicular laser excitation polarization (D(∥)/D(⊥)). The observed spectroscopic trends were used to infer the average chiral angles and edge smoothness of GNRs. Electrical transport and Raman measurements were carried out for individual ribbons to correlate spectroscopic and electrical properties of GNRs.

Metallic Membranes with Subwavelength Complementary Patterns: Distinct Substrates for Surface-Enhanced Raman Scattering

We present a detailed comparison of surface-enhanced Raman spectroscopy (SERS) signals from metallic nanoparticle arrays and their complementary hole arrays. Using an analytical model for local field enhancement, we show that the SERS enhancements of the hole arrays are closely related to their transmission spectra. This trend is experimentally confirmed and characterized by a cos(4 )θ dependence of the SERS signal on the excitation polarization angle θ. The particle arrays, on the other hand, exhibit quite different behavior because of the existence of considerable evanescent modes in the near field. Their maximal local field gains appear at wavelengths generally much larger than their localized surface plasmonic resonant wavelengths.

Regioregularity and Single Polythiophene Chain Conformation

The regioregularity of a conjugated polymer can greatly affect bulk film morphologies and properties. However, it remains unclear how regioregularity affects the conformation of isolated individual chains where interchain interactions are absent. Here, the effect of the regioregularity on the conformations adopted by regioregular (rr-) and regiorandom (rra-) poly(3-hexylthiophene) (P3HT) chains was studied using single molecule fluorescence excitation polarization spectroscopy. While every rr-P3HT chain within an ensemble was found to fold into a highly ordered conformation, single rra-P3HT chains adopted a wide variety of conformations, ranging from highly ordered to isotropic. This distribution is likely due to variations in the positions of nonhead-to-tail linkages of the side-chains along the backbone of the different polymer chains. Molecular dynamics simulation on atomistic models of rr- and rra-P3HT chains supports the effect of regioregularity on the collapsed conformations. These results demonstr...

Experimental Evidence of Plasmophores: Plasmon-Directed Polarized Emission from Gold Nanorod–Fluorophore Hybrid Nanostructures

We show that the fluorescence emission from individual hybrid nanostructures composed of Au nanorod cores and oxazine 725-embedded mesostructured silica shells is strongly polarized, with the degree of polarization being equal to that of the light scattered by the nanorod and varying from 0 to 1 as the longitudinal plasmon resonance wavelength is increased. Our analyses indicate that the interactions of the plasmon resonance of the nanorod with the excitation and emission processes of the fluorophores are temporally separated under unsaturated excitation conditions. The emission polarization is found through electrodynamic calculations to arise from the plasmon-coupled emission instead of the plasmon-enhanced excitation polarization. The emission carries the direction and polarization properties that are essentially determined by the dipolar plasmon of the nanorod antenna. Our results therefore provide direct and concrete evidence for the plasmophore that has been proposed recently for plasmon-enhanced fluorescence.

Influence of plasmon destructive interferences on optical properties of gold planarquadrumers

Arrays of planar symmetric gold quadrumers consisting of a centralnano-disc surrounded by three similar nano-discs belonging to theD3h point group were designed and fabricated. Since the geometrical configuration ofquadrumers is the same as planar trigonal molecules, nano-discs can play the roles ofartificial atoms to study the coupling trends among them. The plasmonic properties of thenano-disc structures are investigated by reflection spectrum measurement andfinite-difference time-domain calculation with good agreement. Plasmon interaction amongthe nano-discs is also studied via a mass–spring coupled oscillator model. A pronouncedFano resonance (FR) is observed for the fabricated nano-discs with inter-disk gaps ofaround 18 nm during light irradiation at normal incidence. Although the obtainedFR is independent of the excitation polarization, the near-field energy spatialdistribution can be flexibly tuned by the polarization direction. This has potentialapplications in nano-lithography, optical switching and nonlinear spectroscopy.

Collisional damping of dipole oscillations in a trapped polariton gas

We study the relaxation dynamics of a trapped polariton gas in the nonlinear regime. We excite the three lowest energy states of the system and observe the time evolution of the polariton density in the momentum space. At a low excitation power, the dynamics is characterized by dipole oscillations of constant amplitude. A damping of these oscillations is observed at a high excitation power. It is attributed to collisional relaxation within the coherent polariton gas. We investigate the dependence of this effect on the excitation power, polarization, and polariton excitonic content to highlight the role of polariton-polariton scattering. The experiments are described in the frame of a Gross-Pitaevskii mean-field theory. We find a good agreement between the theoretical simulations and the experimental observations. Analysis of the theoretical model reveals that multiple parametric scattering and final-state stimulation are responsible for the damping of the oscillations.

Methyl-β-Cyclodextrins Preferentially Remove Cholesterol from the Liquid Disordered Phase in Giant Unilamellar Vesicles

Methyl-β-cyclodextrins (MβCDs) are molecules that are extensively used to remove and to load cholesterol (Chol) from artificial and natural membranes; however, the mechanism of Chol extraction by MβCD from pure lipids or from complex mixtures is not fully understood. One of the outstanding questions in this field is the capability of MβCD to remove Chol from lipid domains having different packing. Here, we investigated the specificity of MβCD to remove Chol from coexisting macrodomains with different lipid packing. We used giant unilamellar vesicles (GUVs) made of 1,2-dioleoylphosphatidylcholine:1,2-dipalmitoylphatidylcholine:free cholesterol, 1:1:1 molar ratio at 27°C. Under these conditions, individual GUVs present Chol distributed into l o and l d phases. The two phases can be distinguished and visualized using Laurdan generalized polarization and two-photon excitation fluorescence microscopy. Our data indicate that MβCD removes Chol preferentially from the more disordered phase. The process of selective Chol removal is dependent on the MβCD concentration. At high concentrations, MβCD also removes phospholipids.

Investigation of the Mechanism of Polar Excitations on Frog Sciatic Nerve by Optical Stimulation

We observed nerve excitations on bullfrog sciatic nerves by an optical stimulation using a Holmium YAG (Ho:YAG) laser with the laser power over 2.0 [W]. The laser power was higher the amplitude of the action potential was bigger. However, when the laser power over 10.0 [W], the laser gave damage to the sciatic nerve and the action potential decreased. Not only the action potential was evoked by the high temperature but also the sciatic nerve was damaged by an overlarge temperature. These results show the possibility that the single wavelength laser can use to position estimation of epileptogenic focus and irreversible blocking of propagation of epilepsy seizure.

Investigation of the Optimum Design for a 10 W Step-down 3-layer Piezoelectric Transformer

This paper presents the electrical properties of a step-down multilayer piezoelectric transformer with square shape fabricated with a piezoelectric ceramic. ATILA, a modeling program, was used to simulate the piezoelectric transformer, and its results were compared to these of practical experiment. A 3D model with the size corresponding to the experiment was created in the ATILA program. Simulated parameters, including the excitation voltage, material factors, polarization, and boundary conditions, and two types of analyses (modal and harmonic) were used to present and calculate the property of piezoelectric transformer. The frequency dependence of the step-down ratio and the input voltage dependence of power properties in the simulation were similar to the experiment results. Through a comparison with experiment, the potential profile in the model is described visually for different loads and frequencies. A power of 10 W was achieved at load of 100 Ω and an input voltage of 60 V, for which the temperature rise was less than 20 ◦C.

Excitation of a one-dimensional evanescent wave by conical edge diffraction of surface plasmon

The experimental observation of a one-dimensional evanescent wave supported by a 90◦ metal edge is reported. Through a measurement of in-plane momenta, we clearly demonstrate the dimensional character of this surface wave and show that it is non-radiative in the superstrate. Excitation conditions, lateral extension and polarization properties of this wave are discussed. Finally, we explore the effect of the surrounding dielectric medium and demonstrate that a single edge can sustain distinct excitations.

Coherence in Metal−Metal-to-Ligand-Charge-Transfer Excited States of a Dimetallic Complex Investigated by Ultrafast Transient Absorption Anisotropy

Coherence in the metal-metal-to-ligand-charge transfer (MMLCT) excited state of diplatinum molecule [Pt(ppy)(μ-(t)Bu(2)pz)](2) has been investigated through the observed oscillatory features and their corresponding frequencies as well as polarization dependence in the single-wavelength transient absorption (TA) anisotropy signals. Anticorrelated parallel and perpendicular TA signals with respect to the excitation polarization direction were captured, while minimal oscillatory features were observed in the magic angle TA signal. The combined analysis of the experimental results coupled with those previous calculated in the literature maps out a plausible excited state trajectory on the potential energy surface, suggesting that (1) the two energetically close MMLCT excited states due to the symmetry of the molecule may be electronically and coherently coupled with the charge density shifting back and forth between the two phenylpyridine (ppy) ligands, (2) the electronic coupling strength in the (1)MMLCT and (3)MMLCT states may be extracted from the oscillation frequencies of the TA signals to be 160 and 55 cm(-1), respectively, (3) a stepwise intersystem crossing cascades follows (1)MMLCT → (3)MMLCT (T(1b)) → (3)MMLCT (T(1a)), and (4) a possible electronic coherence can be modulated via the Pt-Pt σ-interactions over a picosecond and survive the first step of intersystem crossing. Future experiments are in progress to further investigate the origin of the oscillatory features. These experimental observations may have general implications in design of multimetal center complexes for photoactivated reactions where coherence in the excited states may facilitate directional charge or energy transfer along a certain direction between different parts of a molecule.

Affinity of PIP-aquaporins to sterol-enriched domains in plasma membrane of the cells of etiolated pea seedlings

The hypothesis that sterol-enriched domains represent sites of preferred localization of PIP-aquaporins was tested in experiments on plasma membranes isolated from cells of etiolated pea (Pisum sativum L.) seedlings. Plasma membranes were isolated from microsomes by the partition in the aqueous two-phase polymer system and separated into vesicle fractions of different buoyant density by flotation in discontinuous OptiPrep gradient. Two types of plasma membrane preparations were used: one was treated with cold 1% Triton X-100 and the other was not. In untreated preparations, three populations of plasma membrane vesicles were obtained, while in the case of treated preparations, fractions of detergent-resistant membranes (DRM) and solubilized membrane proteins were obtained. In all membrane fractions collected after OptiPrep flotation, the amounts of proteins, sterols, and PIP-aquaporins were determined. The highest sterol content was detected in the membrane fraction with buoyant density 1.098 g/cm3 and in the DRM fraction (1.146 g/cm3). These fractions contained much more PIP-aquaporins than the other ones. Phase state of the lipid bilayer was determined by measuring generalized polarization excitation of fluorescence (GPEX) of laurdan incorporated into the membranes of different fractions. It was revealed that the lipid bilayer of the membranes with density of 1.098 g/cm3 had a higher extent of ordering than that of the fractions with density of ∼1.146 g/cm3. The results indicated that uppermost local concentrations of PIP-aquaporins were associated with tightly packed sterol-enriched domains. Moreover, upon solubilization of plasma membrane with Triton X-100, PIP-aquaporins mainly resided in DRM, thus exhibiting a high affinity to sterols.

Single photon emitters based on Ni/Si related defects in single crystalline diamond

We present investigations on single Ni/Si related color centers produced via ion implantation into single crystalline type IIa CVD diamond. Testing different ion dose combinations we show that there is an upper limit for both the Ni and the Si dose 10^12/cm^2 and 10^10/cm^2 resp.) due to creation of excess fluorescent background. We demonstrate creation of Ni/Si related centers showing emission in the spectral range between 767nm and 775nm and narrow line-widths of 2nm FWHM at room temperature. Measurements of the intensity auto-correlation functions prove single-photon emission. The investigated color centers can be coarsely divided into two groups: Drawing from photon statistics and the degree of polarization in excitation and emission we find that some color centers behave as two-level, single-dipole systems whereas other centers exhibit three levels and contributions from two orthogonal dipoles. In addition, some color centers feature stable and bright emission with saturation count rates up to 78kcounts/s whereas others show fluctuating count rates and three-level blinking.

Polarization of photoluminescence excitation and emission spectra of silicon nanorods within single Si/SiO2 nanowires

AbstractPolarization properties of individual silicon nanowires are studied using an optical micro‐spectroscopy setup equipped with a Fresnel rhomb to rotate the polarization of the exciting laser and the analyzer to characterize the polarization of emitted photoluminescence. The Si nanowire samples are prepared by electron‐beam lithography, plasma etching and oxidation. The fabricated wires are embedded in SiO2 and oriented parallel to the Si substrate. Due to the fluctuating wire diameter (around 5 nm) the very long wires (several tens of μm) are effectively divided into an array of quantum rods (prolate ellipsoids). These structures have strong photoluminescence under UV‐blue excitation at room temperature. The degree of photoluminescence linear polarization of both excitation and emission is very high, between 0.9‐1, and reveals relatively low fluctuations at different spots of the wires. Experimental results are compared with available theoretical models leading to the conclusion that the high polarization degree is mostly due to surface charges (dielectric confinement) with smaller contribution of quantum confinement effects (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Linear Polarization of Photoluminescence Excitation and Emission Spectra of Porous Silicon

We have found a separation of the photoluminescence band caused by a strong spin-orbit splitting of holes in a porous silicon quantum well. The porous silicon has a red-orange photoluminescence (480 ∼ 800 nm; λmax. about 600 nm) due to the quantum-confinement effect. In this research, the existence of sub-bands was newly confirmed by using the polarized excitation and photoluminescence spectroscopy. These sub-bands depended on the harmony of the rotation angle of the excitation and/or emission polarizers, respectively, and the degree of polarization was negative or positive values as a rotation angle of the excitation and emission polarizers. From these results, the quantumconfined band structure of the porous silicon is more concretely explained by using the spin-orbit splitting model. The photoluminescence band of the porous silicon was composed of an electronheavy hole and electron-light hole recombination, and its relaxation process is in good agreement with the electron transition selection rule. Namely, the polarization of the nanoporous material is caused by a strong spin-orbit splitting of the quantum-confined valence band.

Confined Optical Fields in Nanovoid Chain Structures Directly Visualized by Near-Field Optical Imaging

Linearly aligned nanovoid chain structures on Au thin films were fabricated on glass substrates. Two-photon excitation imaging techniques using an aperture-type near-field scanning optical microscope revealed localized optical field distributions due to the individual structures. Confined optical fields due to the void chains were observed at each interstitial gap between voids with excitation polarization parallel to the chain axis. Electromagnetic field simulations of dimeric voids qualitatively matched the experimental results. Under excitation polarization perpendicular to the chain axis, only weak optical fields were observed. Further detailed characteristics of localized optical fields in these systems, including those in relation to Babinet’s principle in optics, were discussed. Our study may open up new possibilities in molecular sensing and photochemistry.